The catecholamines norepinephrine (NE) and dopamine (DA), known inhibitors of insulin secretion, have been identified in the pancreatic islets only semiquantitatively by their histochemical fluorescence spectrum. Using collagenase digestion of golden hamster pancreas, we isolated islets and quantitated the NE and DA content with sensitive and specific radioenzymatic assays. We compared the NE and DA concentrations in islets with the median eminence and cerebral cortex of the golden hamster, tissues known to contain catecholamines. Under basal conditions, NE is in higher concentration than DA in all three tissues and is highest in the islets in a much greater concentration than is found in the median eminence. DA is present in measurable quantities in all three tissues and, in the islets, its concentration is comparable with that in the median eminence.
Treatment of the hamster with a precursor of the two catecholamines, L-dopa, produced a significant accumulation of DA in the islets and median eminence over the basal values. This L-dopa-induced increase in DA was heightened in the islets and reached significantly higher than control levels in the cortex by a pretreatment of the hamster with tranylcypromine (tran), a monomine oxidase inhibitor. Pretreatment with tran alone, however, produced no change in DA over control values.
The NE content of pancreatic islets was not altered by administration of L-dopa, tran, or L-dopa plus tran. The NE content of the median eminence was increased by the administration of tran. L-dopa plus tran did not result in an additional increase in the NE content of the median eminence. The administration of tran plus L-dopa (but not the administration of either agent alone) increased the NE content of the cortex over control levels. The present study suggests that inhibition of insulin secretionafter L-dopa administration is due to an increased concentration of DA rather than NE in the pancreatic islets.